Patent document 1 discloses a white balance adjustment method for a projector device using a plurality of laser light sources that emit different types of color laser light and using a plurality of image forming elements that form images corresponding to the respective colors of the laser light, the method involving disposing respective color sensors for detecting light quantities in the light paths of the laser light sources, and controlling outputs from the laser light sources according to values detected by the sensors. Such white balance adjustment is performed in order to adjust the light quantities having changed due to aged deterioration of the laser light sources or in order to maintain the white balance at a time of a laser output change for light control.
The respective color sensors disposed in the light paths of the laser light sources are provided on the rear surface of a reflection mirror for bending the light paths of the laser light. The reflection mirror reflects most of the indent light, but a part of the incident light passes through the reflection mirror. Since the ratio of the passing light to the incident light is fixed, the light quantities of laser light are detected through sensing of the passing light.